Expression of heterologous DNA sequences in a plant host is dependent upon the presence of an operably linked promoter that is functional within the plant host. Choice of the promoter sequence will determine when and where within the organism the heterologous DNA sequence is expressed. Modifications of the promoter sequences or additional regulatory sequences upstream and/or downstream from the promoter sequence may be included in expression constructs to bring about varying levels of expression of heterologous nucleotide sequences of interest in a transgenic plant.
Frequently it is desirable to modulate the level of expression of a nucleotide sequence of interest along with the temporal and spatial expression of the nucleotide sequence of interest in a plant. For example, increased resistance of a plant to infection by soil- and air-borne pathogens might be accomplished by genetic manipulation of the plant's genome to comprise a tissue-preferred promoter operably linked to a heterologous herbicide-resistance gene or heterologous pathogen-resistance gene. Alternatively, it might be desirable to inhibit expression of a native DNA sequence within a plant's tissues to achieve a desired phenotype. In this case, such inhibition might be accomplished with transformation of the plant to comprise a tissue-preferred promoter operably linked to an antisense nucleotide sequence, such that expression of the antisense sequence produces an RNA transcript that interferes with translation of the mRNA of the native DNA sequence.
Thus, isolation and characterization of promoter sequences that allow varying levels of expression, locations of expression, and inducible expression conditions of heterologous nucleotide sequences of interest in a transgenic plant are needed for genetic manipulation of plants.